When constructing power amplifiers various components must be mounted to a circuit board or substrate. One such component is a power component or power device such as, for instance, a power transistor, also referred to herein as a radio frequency (or “RF”) transistor. When mounting an RF transistor that includes one or more input terminals and one or more output terminals to a circuit board with corresponding input and output signal lines, two factors must be balanced, i.e., the need for a proper electrical coupling of the device to the circuit board must be balanced against the high heat dissipation needs of the device. To provide for optimal electrical performance of the power transistor both the input and the output terminals are typically mounted using a known solder reflow process to a topside of the circuit board, and ideally input and output signal lines to which the transistor terminals are attached are grounded as close as possible to the body of the transistor, thus providing a low inductance electrical match of the transistor to the rest of the circuit. Moreover, a sufficient thermal conduction path must be established between the RF transistor and a heat sink that is typically soldered locally to the underside of the circuit board in an area primarily surrounding the RF transistor.
There are a number of methods used for mounting a power device such as an RF transistor to a circuit board, including a hybrid manufacturing process using fixtures (i.e., a one pass solder reflow process) and a two pass solder reflow process. The hybrid manufacturing process is typically associated with ceramic circuit boards and possibly with carrier plates that serve as heat sinks. Due to the fragility of the substrate, large fixtures are usually required for its alignment and protection during processing. The use of fixtures usually forces manual processing.
One disadvantage of the hybrid manufacturing process is that it is more costly than other manufacturing methods primarily due to the added cost of the fixtures used in the process and also due to the need for a number of manual steps that generate a lower production throughput. An additional disadvantage is that manufacturing with fixtures produces a significant variation in the ground attachment relative to the signal lines to which the transistor terminals are attached.
Turning now to the two pass solder reflow process. During the first pass of the solder reflow process, a plurality of heat sinks are locally coupled to the ground layer of a circuit board in areas primarily surrounding where power components will be mounted. Thereafter, solder is placed in strategic areas on the board, and a plurality of components, including RF transistors, are mounted onto the board in a second pass of the reflow solder process. The RF power transistors are mounted on the circuit board in two regions. First, the input and output terminals of the RF transistor are mounted on the topside of the board. Second, a portion of the RF transistor is located within an aperture in the circuit board so that the RF transistor can also be coupled to at least one heat sink. During the second pass of the reflow process, the components, including the RF transistors are soldered to the circuit board and, in the case of the RF transistors, to the heat sinks.
A primary disadvantage of the two pass reflow process is that it requires one high-temperature reflow pass with a high melting temperature solder alloy, and a second subsequent reflow pass with a lower melting temperature solder allow. The first pass exposes the circuit board to high temperature, which can result in damage such as distortion. The requirement of two independent passes with different solder temperature settings limits manufacturing throughput. The two pass approach also does not lend itself well to no-lead solder because the first temperature needed to attach the heat sinks would have to exceed the elevated no-lead solder reflow temperature. This is a significant disadvantage because no-lead solder attachment may likely become a key product differentiator in the near future since some markets, especially European markets, are moving toward requiring no-lead solder attachment.
Thus, there exists a need for a cost effective method and electrical circuit apparatus wherein components, namely power devices such as RF transistors, may be mounted to a circuit board without the need for fixtures and that is compatible with a single pass solder reflow process that is compatible with, but is not limited to no-lead solder.